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| United States Patent |
5,306,555
|
|
Ramamurthi
, et al.
|
April 26, 1994
|
Aerogel matrix composites
Abstract
An aerogel matrix composite of a bulk aerogel and fibers dispersed within
the bulk aerogel, and a method for preparing the aerogel matrix composite
having the steps: Making a first solution by mixing a metal alkoxide with
an alcohol; making a second solution by mixing an alcohol, water, and
base; mixing the first and second solutions to form a third solution and
mixing fibers therewith; aging the third solution containing the fibers to
obtain a gelled composition; completely submerging the gelled composition
in a liquid suitable for supercritical drying; heating and pressurizing
the gelled composition at a rate between about 75.degree. C. per hour to
about 500.degree. C. per hour until at least the critical temperature and
pressure of at least the major liquid in the gel composition are reached;
maintaining at least the critical temperature and pressure for a time
sufficient to transform the liquid to a supercritical fluid; and reducing
the pressure and temperature to ambient conditions by reducing the
pressure at a rate above 500 psi per hour, and maintaining temperature
above at least the critical temperature until the critical pressure
transition is passed.
| Inventors:
|
Ramamurthi; Sangeeta (West Worthington, OH);
Ramamurthi; Mukund (West Worthington, OH)
|
| Assignee:
|
Battelle Memorial Institute (Columbus, OH)
|
| Appl. No.:
|
904777 |
| Filed:
|
June 26, 1992 |
| Current U.S. Class: |
442/63; 428/292.1; 442/172; 501/12; 516/111 |
| Intern'l Class: |
D04H 001/58 |
| Field of Search: |
252/315.1
501/12
428/289,297
|
References Cited
U.S. Patent Documents
| 2808338 | Oct., 1957 | Bruno et al. | 252/62.
|
| 2945817 | Jul., 1960 | Goldblum | 252/62.
|
| 3629116 | Dec., 1971 | Gartner et al. | 252/62.
|
| 3869334 | Mar., 1975 | Hughes et al. | 252/62.
|
| 4327065 | Apr., 1982 | von Dardel et al. | 423/338.
|
| 4402927 | Sep., 1983 | von Dardel et al. | 423/335.
|
| 4432956 | Feb., 1984 | Zarzycki et al. | 423/338.
|
| 4447345 | May., 1984 | Kummermehr et al. | 252/62.
|
| 4610863 | Sep., 1986 | Tewari et al. | 423/338.
|
| 4667417 | May., 1987 | Graser et al. | 34/9.
|
| 4911775 | Mar., 1990 | Kuma et al. | 156/208.
|
| Foreign Patent Documents |
| 0018955 | Nov., 1980 | EP.
| |
| 0382310 | Aug., 1990 | EP.
| |
| 3844003 | Mar., 1990 | DE.
| |
| 2141418 | Dec., 1984 | GB.
| |
Other References
Aerogels; Jochen Fricke; Scientific American; May 1988; pp. 92-97.
Fibre Reinforce Glass and Ceramic Composites; A. R. Hyde; GEC Journal of
Research, vol. 6; 1; 1988, pp. 44-49.
Aerogels and Related Porous Materials; H. D. Gesser et al; Chem. Rev.; 89;
1989; pp. 765-788.
Status of Continuous Fiber-Reinforced Ceramic Matrix Composite Processing
Technology; J. R. Strife et al; Ceram. Eng. Sci. Proc.; 11[7-8]; 1990; pp.
871-919.
Fabrication of Particulate, Platelet, Whisker and Continuous Fibre
Reinforced Glass, Glass Ceramic and Ceramic Materials; A. R. Hyde et al;
Br. Ceram. Proc.; 45; (Fabr. Technol.); 1989; pp. 221-227.
Aerogels, a Source of Adsorbents, Insulators, Catalysts and Ceramics; S. J.
Teichner; Revue de Physique Appliquee; Colloque C4; Supplement au
n.degree.4; Tome 24; Avril 1989; pp. C4-1 through C4-6.
Advanced Composites for High Temperatures (A Study on a Suitable
Fabrication Process); E. Fitzer et al; Metals Materials and Processes;
vol. 2; No. 3; 1990; pp. 179-200.
|
Primary Examiner: Bell; James J.
Attorney, Agent or Firm: Wiesmann; Klaus H.
Parent Case Text
This application is a continuation-in-part of copending application Ser.
No. 07/761,577, filed Sep. 18, 1991, now abandoned the disclosure of which
is incorporated by reference as if completely rewritten herein.
Claims
We claim:
1. A method for preparing an aerogel matrix composite comprising:
a. preparing an aerogel precursor;
b. mixing fibers with the aerogel precursor;
c. aging the aerogel precursor containing the fibers to obtain a gelled
composition;
d. completely submerging the gelled composition in a liquid suitable for
supercritical drying;
e. heating and pressurizing the gelled composition at a rate between about
75.degree. C. per hour to about 500.degree. C. per hour until at least the
critical temperature and pressure of a liquid in the gel composition are
reached;
f. maintaining at least the critical temperature and pressure for a time
sufficient to transform the liquid to a supercritical fluid; and
g. reducing the pressure and temperature to ambient conditions by reducing
the pressure at a rate above 500 psi per hour, and maintaining the
temperature above at least the critical temperature until the critical
pressure transition is passed.
2. The method of claim 1, wherein the fibers are selected to have a thermal
conductivity less than about 1.0 W/m.degree.K.
3. The method of claim 1, whereby there is formed an aerogel matrix
composite that is substantially crack free.
4. The method of claim 1, whereby there is formed an aerogel matrix
composite having substantially no volume shrinkage.
5. The method of claim 1, whereby there is formed an aerogel matrix
composite having a volume shrinkage less than about 1 percent.
6. The method of claim 1, whereby the heating in step (e) is at a rate
between about 1OO.degree. C. per hour to about 150.degree. C. per hour.
7. The method of claim 1, whereby the pressurizing in step (e) includes
pressurizing above ambient pressure prior to applying heat.
8. The method of claim 7, whereby the pressurizing is above about 400 psig
prior to applying heat.
9. A monolithic aerogel matrix composite formed by the method of claim 1.
10. A method for preparing an aerogel matrix composite comprising:
a. making a first solution by mixing a metal alkoxide with an alcohol;
b. making a second solution by mixing an alcohol, water, and base;
c. mixing the first and second solutions to form a third solution and
mixing fibers therewith;
d. aging the third solution containing the fibers to obtain a gelled
composition;
e. completely submerging the gelled composition in a liquid suitable for
supercritical drying;
f. heating and pressurizing the gelled composition at a rate between about
75.degree. C. per hour to about 500.degree. C. per hour until at least the
critical temperature and pressure of at least a major liquid in the gel
composition are reached;
g. maintaining at least the critical temperature and pressure for a time
sufficient to transform the liquid to a supercritical fluid; and
h. reducing the pressure and temperature to ambient conditions by reducing
the pressure at a rate above 500 psi per hour, and maintaining temperature
above at least the critical temperature until the critical pressure
transition is passed.
11. The method of claim 10, wherein the alcohols of steps (a) and (b) and
the liquid of step (e) are all the same alcohols.
12. The method of claim 10, whereby there is formed an aerogel matrix
composite that is substantially crack free.
13. The method of claim 10, whereby there is formed an aerogel matrix
composite having substantially no volume shrinkage.
14. The method of claim 10, whereby there is formed an aerogel matrix
composite having a volume shrinkage less than about 1 percent.
15. The method of claim 10, whereby the heating in step (f) is at a rate
between about 1OO.degree. C. per hour to about 150.degree. C. per hour.
16. The method of claim 10, whereby the pressurizing in step (f) includes
pressurizing above ambient pressure prior to applying heat.
17. The method of claim 16, whereby the pressurizing is above about 400
psig prior to applying heat.
18. The method of claim 10, comprising:
(1) in the first solution the metal alkoxide is Si(OCH.sub.3).sub.4 and the
alcohol is methyl alcohol;
(2) in the second solution the alcohol is methyl alcohol and the base is
ammonia; and
(3) the submerging liquid is methyl alcohol.
19. A monolithic aerogel matrix composite formed by the method of claim 10.
20. An aerogel matrix composite comprising:
a. a monolithic aerogel; and
b. fibers dispersed within the monolithic aerogel.
21. The composite of claim 20, comprising fibers randomly distributed
throughout the monolithic aerogel.
22. The composite of claim 20, comprising fibers in the form of a mat or
sheet.
23. The composite of claim 20, comprising fibers having a thermal
conductivity of less than 1.0 W/m.degree.K.
Description
FIELD OF THE INVENTION
This invention relates to the preparation and application of aerogel matrix
composites with improved handleability at the gel and aerogel stage,
greater range of flexibility, comparable thermal insulation, and other
improvements in aerogel properties. The invention provides for process
conditions that allow the preparation of aerogels at a much faster rate
than previously possible. The invention has utility in various kinds of
thermal insulation and acoustic insulation applications. Examples of these
applications include insulation for refrigeration, appliance, floor, wall
and home, airplane body, boats and other marine equipment, and electrical
equipment. The same concept of composition and processing is applicable in
gas filtration membranes, catalytic supports, and in other applications
where nontransparent materials can be used.
BACKGROUND AND RELATED ART
Escalating energy costs have lead to increased efforts in exploring more
effective insulation materials for windows, houses, water heaters, as well
as other appliances and equipment. In recent years, aerogels have been
suggested as prospective insulation materials for these applications.
Aerogels are a unique class of ultra-low-density (0.1-0.2 g/cc) materials
with 90-99 percent porosity; the high porosity, intrinsic pore structure,
and low density makes aerogels extremely valuable materials for these
applications. Conventional aerogel preparation techniques involve the
preparation of a gel that is subsequently dried under supercritical
conditions. Monolithic aerogels prepared using these techniques disclosed
in the literature are fragile and the preparation process involves complex
time-consuming steps.
Alternatively, aerogel powder-fiber compacts (APFC) have also been
suggested as prospective thermal insulation materials. APFCs are prepared
by a process involving the prefabrication of several ingredients (aerogel
powder, fibers, binders), which are later mixed together and compacted to
form insulation boards or cloths. The preparation of APFCs, however,
requires large amounts of aerogel powder and involves a significant number
of steps in the fabrication process. Additionally, the insulating
properties of the typical APFCs disclosed previously are inferior to those
of the fragile, monolithic aerogels.
Related U.S. Pat. Nos. include: 2,808,338; 2,945,817; 3,629,116; 3,869,334;
4,402,927; 4,447,345; and 4,610,863. Related European patents include: EP
018,955 (1980) and EP 382,310 (1990). British patent application 2,141,418
discloses a process for the production of carbon containing materials
having ultrafine grains. The patent further discloses producing a very
dense body containing carbon by placing carbon fibers in a silica aerogel
and chemically depositing carbon in the vapor phase into the silica
aerogel to form a dense body with high thermal conductivity.
BRIEF DESCRIPTION OF THE INVENTION
The invention deals with inorganic aerogel matrix composites (AMCs) that
have enhanced strength, decreased sensitivity to moisture, good thermal
insulation values, and rigidity or flexibility based on the required
application, and no volume shrinkage during supercritical drying. These
desirable characteristics are obtained through formulation modifications
and processing improvements such as molding, processing, and handling
techniques. The invention also includes a reduction in process time, which
is considerably shorter compared to the prior art, making the process more
viable for commercial applications.
A general embodiment of a method for preparing an aerogel matrix composite
typically comprises preparing an aerogel precursor; mixing fibers with the
aerogel precursor; aging the aerogel precursor containing the fibers to
obtain a gelled composition; completely submerging the gelled composition
in a liquid suitable for supercritical drying; heating and pressurizing
the gelled composition at a rate between about 75.degree. C. per hour to
about 500.degree. C. per hour until at least the critical temperature and
pressure of a liquid in the gel composition are reached; maintaining at
least the critical temperature and pressure for a time sufficient to
transform the liquid to a supercritical fluid; and reducing the pressure
and temperature to ambient conditions by reducing the pressure at a rate
above 500 psi per hour, and maintaining the temperature above at least the
critical temperature until the critical pressure transition is passed. In
one typical embodiment the fibers are selected to have a thermal
conductivity less than about 1.0 W/m.degree.K. The typical aerogel matrix
composite formed is substantially crack free, having substantially no
volume shrinkage, where the volume shrinkage less than about I percent.
The heating rate may be above 50.degree. C. per hour to about 150.degree.
C. per hour, to 500.degree. C. per hour. In another embodiment the
pressurizing includes pressurizing above ambient pressure prior to
applying heat, and the pressurizing may be above about 400 psig prior to
applying heat. A monolithic aerogel matrix composite is typically formed
by the above method.
Another typical embodiment of the method for preparing an aerogel matrix
composite comprises making a first solution by mixing a metal alkoxide
with an alcohol; making a second solution by mixing an alcohol, water, and
base; mixing the first and second solutions to form a third solution and
mixing fibers therewith; aging the third solution containing the fibers to
obtain a gelled composition; completely submerging the gelled composition
in a liquid suitable for supercritical drying; heating and pressurizing
the gelled composition at a rate between about 75.degree. C. per hour to
about 500.degree. C. per hour until at least the critical temperature and
pressure of at least a major liquid in the gel composition are reached;
maintaining at least the critical temperature and pressure for a time
sufficient to transform the liquid to a supercritical fluid; and reducing
the pressure and temperature to ambient conditions by reducing the
pressure at a rate above 500 psi per hour, and maintaining temperature
above at least the critical temperature until the critical pressure
transition is passed. The alcohols may all be the same alcohols or
different. The typical aerogel matrix composite formed is substantially
crack free, having substantially no volume shrinkage, where the volume
shrinkage less than about 1 percent. The heating rate may be above
50.degree. C. per hour to about 150.degree. C. per hour, to 500.degree. C.
per hour. In another embodiment the pressurizing includes pressurizing
above ambient pressure prior to applying heat, and the pressurizing may be
above about 400 psig prior to applying heat. A monolithic aerogel matrix
composite is typically formed by the above method. In another embodiment
the method comprises the steps where: (1) in the first solution the metal
alkoxide is Si(OCH .sub.3).sub.4 and the alcohol is methyl alcohol; (2) in
the second solution the alcohol is methyl alcohol and the base is ammonia;
and (3) the submerging liquid is methyl alcohol.
In another typical embodiment an aerogel matrix composite is formed that
comprises: a monolithic aerogel; and fibers dispersed within the
monolithic aerogel. The fibers may be randomly distributed throughout the
monolithic aerogel or in the form of a mat or sheet, or a plurality of
mats or sheets. The fibers preferably have a thermal conductivity of less
than 1.0 W/m.degree.K. The aerogel matrix composite is substantially crack
free with substantially no volume shrinkage. The volume shrinkage is less
than about 1 percent.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic that illustrates the salient features and major
differences between the processing route described in this invention for
aerogel matrix composites (AMCs) and those disclosed in the prior art for
conventional aerogels and aerogel powder-fiber compacts (APFCs).
FIG. 2 is a graph that shows reactor temperature, T, in .degree. C. (left
hand ordinate) and pressure, P, in psig (right hand ordinate) plotted
versus time, t, in minutes (abscissa) for a typical batch processing run
of an aerogel matrix composite. The asterisk in the figure refers to the
point in the run at which the reactor depressurization was begun.
FIG. 3 is a graph showing a plot (white dots) of elastic modulus, E, in psi
(left hand ordinate) and a plot (black dots) of R-value (right hand
ordinate) for a set of AMC samples (A,B,C,D) produced from the process
described in FIG. 1. The samples A,B,C,D are described in Table I and
accompanying text.
FIG. 4 is a comparison between the thermal conductivity of AMCs of the
present invention (A) and conventional aerogels (B) at temperatures higher
than 26.degree. C. In the figure, T is the temperature in .degree.K.,
.DELTA.k is the change in thermal conductivity in W/m.degree.K., and
.DELTA.R is the change in R value. The R value scale is not linear and is
given for reference only.
DETAILED DESCRIPTION OF THE INVENTION
The high porosities and low bulk densities of aerogels make them
potentially very effective insulation materials, with reported thermal
conductivities of monolithic aerogels on the order of 0.02 W/m.degree.K.
at ambient pressure in air. However, monolithic aerogels are extremely
fragile and have low elasticities, consequently limiting their utility in
insulation and most other applications. Similarly, APFCs also require
involved fabrication and the resultant materials have relatively low
thermal insulation values compared with monolithic aerogels.
To overcome these drawbacks, aerogel matrix composites (AMCs) with a
variety of unique characteristics have been prepared. The AMCs consist of
a bulk or monolith aerogel matrix prepared by supercritical drying of a
gel with a fiber-type reinforcement. As used herein monolithic aerogel has
the same meaning as bulk aerogel to define an aerogel produced as a
monolithic structure directly from a gel, and not produced from an aerogel
powder -or other similar discreet aerogel material that is later processed
by compression and/or heat to produce a larger aerogel structure. The
aerogel matrix can be comprised of TiO.sub.2, SiO.sub.2, ZrO.sub.2, and
other similar sol-gel oxides and their composites. These sol-gel matrices
could also include any of the above oxides plus CuO, NiO, Fe.sub.2
O.sub.3, ZnO, or metals for catalytic applications. To enhance the
mechanical properties of these sol-gel derived monolithic aerogels, the
gel matrices were reinforced with long or short fibers of varying
thicknesses, whiskers, mineral wool, glass wool, and particles. The
composition of the reinforcing material may include oxides such as
SiO.sub.2 and Al.sub.2 O.sub.3 (fibers, whiskers, and wools) and carbon,
metals, and a variety of oxides (particles). These reinforcements also
provide opacification to the radiative heat flow. In addition to these
reinforcements, pigments like TiO.sub.2 can also be added to AMCs for
further opacification. The fibers preferably have a thermal conductivity
of less than about 1.0 W/m.degree.K. and most preferably less than 0.5
W/m.degree.K. These conductivities will provide for the desired reduction
in heat flow while simultaneously providing structural support. By
utilizing fibers having low thermal conductivity the insulating properties
of the aerogel are maintained. Examples of preferred fibers are glass wool
(k=4.2.times. 10.sup.-2 W/m.degree.k.), rock wool (k=3.7.times.10.sup.-2
W/m.degree.k.) . The length of the fibers may be any that provides the
desired properties. Thus, fibers between 25 microns to several inches in
length may be used. Longer fibers as well as long fibers running the
length of the aerogel or having no termination within the aerogel may be
used. The fibers may be randomly distributed or oriented. They may also be
in the form of individual fibers, bundles of fibers, mats or sheets, woven
or unwoven, as needed in the particular application.
The following general procedure is representative of the invention.
Fiber-gel compositions are prepared and transferred to an autoclave, where
they were submerged in the minimum required volume of the corresponding
liquids used for the process. The samples are pressurized to 200-1500 psi
(1.38-10.3 MPa) at room temperature with an inert gas. Gases such as
nitrogen gas and the like may be used. Then the temperature is raised at
50.degree.-500.degree. C. per hour, preferably above 75.degree. C. per
hour, and most preferably above 1OO.degree. C. per hour. This temperature
increase is accompanied by an increase in the autoclave pressure that can
also be at a rate of 200-1500 psi (1.38-10.3 MPa) per hour and preferably
above 500 psi (3.43 MPa) per hour. Higher rates of heating and
pressurizing are preferred and are possible in these samples as compared
with the prior art because of the presence of fibers in the gel
composition. The gel stabilization method thus provides substantial
increase in the mechanical strength of the composite gels, allowing them
to resist cracking during gelation, and disintegration and shrinkage even
at the faster heating and pressurizing processes used during supercritical
drying. To maintain a desired maximum autoclave pressure, fluid vapors can
be bled off as the pressure increases with heating. The final temperature
and the pressure in the autoclave vessel is held between
245.degree.-275.degree. C. and 1400-2500 psi (9.66-17.25 MPa),
respectively. After keeping the gels above the critical point of the
liquid for a SCD time of 15 minutes to 2 hours, the pressure is released
at a rate of 500-2000 psi (3.45- 13.75 MPa) per hour, while maintaining
the temperature above the critical point of the liquid. The material is
then cooled to room temperature. The liquid vapors contained in the
compressed volume of the autoclave can be condensed and collected for
reuse as the system is depressurized. Higher rates of cooling and
depressurizing are preferred and are possible in these samples as compared
with the prior art because of the presence of fibers in the gel
composition. The gel stabilization method provides a substantial increase
in the mechanical strength of the composite gels, allowing them to resist
cracking during cooling, and disintegration and shrinkage even at the
faster cooling and depressurizing rates. The supercritical drying
processes can be successfully completed within a short duration
particularly when the higher ranges of heating, pressurizing, cooling, and
depressurizing are used. This provides a substantial improvement over
previous processes when a substantially crack-free monolith is desired.
More specifically, the oxide matrix gels can be prepared from their
corresponding alkoxides (e.g. Si(OCH.sub.3).sub.4, Si(OC.sub.2
H.sub.5).sub.4, Ti(O--OC.sub.3 H.sub.7).sub.4 Zr(OC.sub.3 H.sub.7).sub.4)
in alcohol . The alkoxides can be hydrolyzed with additives such as HCl,
HNO.sub.3, HF, CH.sub.3 COOH, NH.sub.4 OH, NaOH, KOH, and other organic
acids and bases (e.g. (C.sub.2 H.sub.5).sub.3 N) and reinforced with
fibers, etc. while in the flowable state. As the polymerization process
proceeds the solution hardens into a fiber-gel composition. The fiber-gels
are placed in an autoclave and submerged within the corresponding alcohol
(e.g. methyl, ethyl, isopropyl alcohols) . The vessel is then pressurized
to 100-1500 psi (0.7-10.3 MPa) at room temperature and then the
temperature is increased to 250.degree.-260.degree. C. The increase in
temperature causes an increase in pressure to 1400-2300 psi (9.7-15.9
MPa). After holding the critical temperature and pressure for 15-60
minutes, alcohol is bled out slowly keeping the temperature above the
critical point of alcohol. After the pressure is dropped down to 23 psi
(0.2 MPa), the heat is turned off and the samples were removed. These
conditions result in reducing the process time to 3-7 hours and less which
is significantly less than the prior art processes.
FIG. 1 illustrates schematically the salient features of the processing
route for the preparation of the AMCs, as well as the significant
differences between this route and those disclosed previously for
conventional aerogels and APFCs. Generally the materials used to make the
aerogel are not limited to the specific starting material listed above but
could also include inorganic salt, metal organic, organometallic and the
like precursors of Si, Ti, Zr and other metals.
The resultant aerogel matrix composites were characterized by a
Quantachrome Autosorb-1 BET instrument for surface area, porosity, and
pore size distribution, by scanning electron microscopy for
microstructure, by ASTM C518-85 method (Steady-state heat flux
measurements and thermal transmission properties by Holometrix model
Rapid-K heat flow meter) for thermal conductivities, and by Instron
mechanical testing equipment for compressive strength, tensile strength,
bend strength and elastic modulus. These results indicated that compared
with conventional monolithic aerogels, the AMCs had a range of flexibility
(rigid to flexible), improved mechanical properties, and increased
solvent/moisture resistance without diminishing the thermal insulation
properties.
As a result of these characteristics, AMC gels and aerogels are
significantly easier to handle compared with conventional aerogels
eliminating the need for special gel handling techniques. In addition to
the improved mechanical properties of the AMCs, the process time for their
fabrication has been considerably reduced to within 3-7 hours. Hence,
these property and process improvements make AMCs more viable candidates
for production and commercialization than monolithic aerogels.
In addition, AMCs exhibit better properties, such as thermal insulation and
elastic modulus, than those disclosed for APFCs and require a
comparatively simpler processing route. The advantages of the AMC
processing route over the APFC process include a reduction in the number
of steps involved in fabricating the final insulation material and in the
amount of aerogel material needed per unit insulating volume.
Along with the above unique properties, AMCs have certain advantages over
other existing insulation material including the fact that they are
non-CFC materials (chloro-fluoro carbons, i.e. ozone depleting compounds),
are nonflammable, are stable up to 500.degree.-700.degree. C. (beyond
which sintering starts while maintaining the nonflammable character and no
toxic fumes are produced at higher temperatures), have much longer life
times then CFC blown polyurethane, insulation foams.
Similar to conventional aerogels, the thermal insulation values of AMCs are
enhanced (2-3 times) as the pressure in the bag containing AMC is reduced
to 1/8 or 1/10 of atmospheric pressure. Hence, the AMCs can be used under
soft vacuum (0.1-0.2 atm) for applications requiring higher insulating
properties. In contrast to conventional aerogels, however, AMCs are better
thermal insulator at higher temperatures as shown by the modelling results
in FIG. 4. In this figure T is the temperature in .degree.K., .DELTA.k is
the change in thermal conductivity in W/m.degree.K., and .DELTA.R is the
change in R value. The R value scale is not linear and is given for
reference only. With increasing temperature the contribution of the
radiation factor to the heat flow increases. Hence, the opacification by
fibers in AMCs becomes an increasingly important factor.
EXAMPLE 1-A
For the discussion below please refer to Tables I and II that illustrate
various batches for this example. In the tables the following
abbreviations are used P-pressure, T-temperature, SCD-supercritical
drying, and V-volume.
TABLE I
______________________________________
Part A
Weight Per-
Percent Density
Surface Area
Material
cent Fibers
Shrinkage (g/cm.sup.3)
(m.sup.2 /g)
______________________________________
A.sup.a
9 <1 0.12 219
B.sup.a
12 <1 0.11 203
C.sup.b
12 <1 0.12 203
D.sup.a
16 <1 0.12 182
E.sup.a
21 <1 0.13 147
F.sup.c
0 5.15 0.05-0.15
500-800
G.sup.d
55 Not 0.35 Not
Applicable Available
______________________________________
TABLE I
__________________________________________________________________________
Part B
R-value
Elastic
Bend
k (hr .multidot. ft.sup.2 .multidot. F/
Modulus
Strength
Porosity
Material
(W/m .degree.K.)
BTU .multidot. inch)
MPa (psi)
MPa (psi)
(%)
__________________________________________________________________________
A.sup.a
0.020 7.2 5.8(838)
0.05(7)
94.5
B.sup.a
0.021 7.0 6.9(1003)
0.12(16.8)
95.0
C.sup.b
0.018 8.0 15.4(2236)
0.13(18.9)
94.5
D.sup.a
0.019 7.3 15.2(2200)
0.19(27.1)
94.5
E.sup.a
0.019 7.3 93.4(13546)
0.26(38.4)
94.0
F.sup.c
0.018-0.021
7-8 <2(<290)
not --
bendable
G.sup.d
0.026 5.4 2.56(370)
0.17(25.0)
--
__________________________________________________________________________
.sup.a Weaved
.sup.b Encased, sandwich sample in contrast to layered sample (see Exampl
1A for details)
.sup.c Conventional aerogel (SiO.sub.2 noncomposite)
.sup.d Aerogel powderfiber compacts (APFCs) (averaged values)
.sup.e All of the samples were disks
Rigid varieties of AMCs were prepared by supercritically drying a silicate
sol-gel solution reinforced with varying loadings of pyrex glass wool
(Corning SiO.sub.2 fibers, 8 .mu.m diameter). The silicate sol-gel
solution was prepared based on the following equation:
Si(OCH.sub.3).sub.4 +5 H.sub.2 +11.1 CH.sub.3 OH+0.0036 NH.sub.4
OH.fwdarw.SiO.sub.2 +CH.sub.3 OH
Solution A was made by mixing 30 parts (parts are defined as volume ratios)
CH.sub.3 OH, 8.73 parts distilled H.sub.2 O, and 0.38 parts NH.sub.4 OH
(29.3 weight percent in H.sub.2 O) solution. Solution B was prepared by
mixing 15 parts CH.sub.3 OH and 14.73 parts Si(OCH.sub.3).sub.4 (TMOS).
Solution A was added to solution B and was stirred at room temperature
resulting in a sol-gel solution that is flowable for a brief period
following mixing. Silica fibers, cut to 4-6 inches (10.2-15.2 cm) in
length, were laid in a thin layer in a silicone rubberized mold. Then a
small amount of sol-gel solution was poured and a layer of silica fibers
was overlaid at an angle of 90.degree. to the earlier layer. The alternate
layers of fibers and sol-gel solution resulted in a fiber-gel composition
with a weaved silica fiber mat (see Table I). In other cases (Table I),
fiber-gel compositions consisting of a gel contained between top and
bottom layers of fiber mats were prepared. In some AMC samples, ready-made
fiber mats were used as facers on the surface of the reinforced samples.
In still other samples, vertical (to the sample thickness) and random
orientations of the fibers in the gel were also prepared. The silicate
solution in the samples hardened into gel in less than 2 minutes. The
fiber-gel samples were prepared in various dimensions and shapes. The most
common examples included 6 to 7 inch (15.2-17.8 cm) discs with 0.5 to 1
inch (1.27-2.54 cm) thickness (Samples Table I-A to E) and 12
inch.times.12 inch.times.1 inch (30.5.times.30.5.times.2.5 cm) tiles were
prepared and tested with similar results. The fiber-gel compositions were
handled very easily in contrast to conventional silica gels that are
sensitive to extensive internal and external cracking from vibrations.
TABLE II
______________________________________
Part A
Initial P P. Ramp
Max. P.
(psi) T. Ramp (psi/hr)
(psi) Max. T
Sample Run
{MPa} (.degree.C./hr)
{MPa/hr}
{MPa} (.degree.C.)
______________________________________
R1 650 75 300 1500 250
(repeated 5
{4.48} {2.07} {10.3}
times)
R2 600 130 1200 1500 255
{4.14} {8.28} {10.3}
R3 600 96 400 2200 252
{4.14} {2.76} {15.17}
R4 600 80 700 2300 260
{4.14} {4.83} {15.86}
R5 600 125 900 2300 260
{4.14} {6.21} {15.86}
R6 1200 75 500 2000 255
{8.28} {3.45} {13.79}
______________________________________
TABLE II
______________________________________
Part B
P. Release Liq. V.
SCD Time (psi/hr) Release
Total Time
Sample Run
(hr) {MPa/hr} (L/hr) (hr)
______________________________________
R1 0.5 1500 3-12 5.3
(repeated 5 {10.3}
times)
R2 0.5 1200 10 3.6
{8.28}
R3 1 900 -- 7.7
{6.21}
R4 1 1400 -- 4.8
{9.66}
R5 1 1600 -- 4.6
{11.03}
R6 1 700 -- 6.0
{4.83}
______________________________________
Referring now to Table II-Parts A and B the following procedure was used to
obtain the materials. The fiber-gel compositions were removed from the
molds and transferred to an autoclave, where they were submerged in the
minimum required volume of the corresponding alcohols within
liners/containers. The samples were pressurized to 600-1200 psi (4.1-8.3
MPa) at room temperature with nitrogen gas. Then the temperature was
raised at 75.degree.-130.degree. C. per hour. This temperature increase
was accompanied by an increase in the autoclave pressure at a rate of
500-1200 psi(3.4-8.28 MPa) per hour. Higher rates of heating and
pressurizing were possible in these samples as compared with the prior art
because of the presence of fibers in the gel composition. The gel
stabilization method thus provides substantial increase in the mechanical
strength of the composite gels, allowing them to resist cracking during
gelation, and disintegration and shrinkage even at the faster heating and
pressurizing processes used during supercritical drying. To maintain a
desired maximum autoclave pressure, methanol and nitrogen vapors were bled
off as the pressure increased with heating. The final temperature and the
pressure in the autoclave vessel was held between 250.degree.-260.degree.
C. and 1500-2300 psi (10.3-15.9 Ma), respectively. After keeping the gels
above the critical point of methanol for SCD time 30 minutes to 1 hour,
the pressure was released at a rate of 700-1500 psi (4.83-10.3 MPa) per
hour, while maintaining the temperature above the critical point of
methanol. The methanol vapors contained in the compressed volume of the
autoclave were condensed and collected for reuse at a rate of 3-12 L/hr.
as the system was depressurized. The supercritical drying processes were
successfully completed within a duration of 3-7 hours. This is a
substantial improvement over previous processes where 24 hours to 20 days
of curing and processing are required to prepare a substantially
crack-free monolith.
The fiber-gel compositions can also be dried by using CO.sub.2 or other
compatible supercritical fluids by exchanging the alcohol with the
selected fluid and using the corresponding critical temperature and
pressure of this fluid above their critical points. The samples (Samples A
to E in Table I) prepared by alcohol supercritical drying did not require
curing at an elevated temperature after the drying process. In comparison,
samples where alcohol was first exchanged with liquid CO.sub.2 and then
dried at the critical temperature and pressure of CO.sub.2 required curing
at higher temperatures (250.degree.-500.degree. C). The alcohol
supercritical drying process resulted in more robust aerogels with
significantly shorter processing time as compared with the CO.sub.2
exchange and drying process. With increased sample thicknesses the
exchange and drying process using liquid CO.sub.2 was substantially more
time-consuming.
The samples (Samples A to E) thus formed were about 6 inches (15.2 cm) in
diameter and about 0.5 inches (1.3 cm) in thickness. The rigid AMCs showed
very little (<1 percent) or no volume shrinkage in the transformation from
the gel to the solid state. In contrast, conventional aerogels made from
nonreinforced gels exhibit between 5-15 percent volume shrinkage. The bulk
densities of the AMCs varied between 0.11-0.14 g/cc, surface areas were
between 120-400 m.sup.2 /g, and porosities were in the range of 92-96 to
94-95 percent. The low densities and high porosities- of AMCs result from
no volume shrinkage during the drying process due to the presence of
fibers in the fiber-gel compositions. The AMC samples had good thermal
insulation properties, with thermal conductivities ranging from 0.014 and
0.021 W/m.degree.K. These thermal conductivities are equivalent to 0.125
and 0.143 BTU.inch/hr.ft.sup.2..degree.F. with R values in the range of
7-8 per inch. The mechanical properties of the aerogel matrix composites
were determined to be a substantial improvement over conventional aerogels
prepared from nonreinforced gels. In particular, the elastic modulus of
these samples ranged between 800-14,000 psi (5.5-96.5 MPa).
EXAMPLE 1-B
Details and results of one specific batch of experiments using the above
procedure are disclosed below:
Fiber-stabilized SiO.sub.2 -methanol gels were prepared using the procedure
outlined earlier in this example. Fiber-gel compositions containing fiber
weight percents ranging from 9-21 percent (in the AMCs) were prepared and
supercritical drying conditions shown in FIG. 2 were employed. Referring
now to FIG. 2, reactor temperature, T, in .degree.C. (left hand ordinate)
and pressure, P, in psig (right hand ordinate) are plotted versus time, t,
in minutes (abscissa) for batch processing of an aerogel. Conditions were
as follows: Temperature 250.degree. C., pressure 1500 psig (10.3 MPa)
maximum, depressurization beginning at 240 minutes, and total run time of
5 hours and 8 minutes. As indicated in FIG. 2, the autoclave temperature
increased at a rate of approximately 100.degree. C. per hour, and the
pressure increased at a rate of approximately 500 psi (3.5 MPa) per hour.
Supercritical conditions of 1450-1500 psi (10-10.3 MPa) and
250.degree.-260.degree. C. were maintained for 1 hour. The autoclave was
depressurized at about 1400-1500 psi (9.7-10.3 MPa) per hour, with the
temperature maintained between 250.degree.-260.degree. C. The test run was
thus processed within a total duration of about 5 hours. It is apparent
that this time can be reduced.
The properties of the AMCs produced from this batch are given in Table
I-Parts A and B including the volume shrinkage, bulk densities, surface
areas, thermal conductivities, R values, bend strengths, and elastic
moduli.
Table I-Part A shows that the AMCs prepared by the procedure disclosed had
little (<1 percent) or essentially no volume shrinkage from the gel to the
AMC. This useful result arises from the stabilizing presence of fibers in
the gel state before and during the supercritical drying process. The bulk
densities of these AMCs ranged from 0.11-0.13 g/cc. These densities are
comparable to those of conventional aerogels and result from the minimal
volume changes introduced by the addition of fibers to gel volume. Table I
also shows the range of surface areas of the various AMCs measured by the
BET method. The surface areas were found to decrease with an increase in
weight percent fibers in the AMC, as the fibers added have low surface
area of <1 m.sup.2 /g. However, the AMCs still have very high surface
areas suitable for a variety of applications.
Referring now to Table I-Part B the thermal conductivities k, where k is in
W/m.degree.K., and R values of the AMCs prepared were measured to be
between 0.018-0.021 W/m.degree.K. and 7-8 per inch, respectively. The
increase in weight percent of silica fibers with R values of 3 per inch in
the AMCs (from 9.0 percent to 21.0 percent) did not decrease the thermal
insulation values (R values) of the AMCs. When the weight percent of
fibers changed from 8 to 21 percent the percent volume changes were
minimal (i.e. 0.4 to 1.24 percent). Since most of the AMC was still
composed of aerogel material, the change in thermal insulation value was
negligible. Similarly, the orientation of the fibers in the AMC did not
affect thermal conductivities. This study indicates that rigid and
flexible AMCs will have comparable or similar R values, even though
flexible AMCs may contain larger amounts of silica fiber (>35 percent, see
Example 2) as compared to rigid AMCs (<25 percent). This observation opens
up a wide range of applications for both rigid and flexible AMCs.
Table I-Part B shows the improvement in the mechanical properties of these
AMCs with increase in weight percent fiber. As the weight percent fibers
increased from 9 percent to 21 percent the elastic moduli increased
exponentially from 838 to 13,546 psi (5.8-93.4 MPa) . These data are shown
graphically in FIG. 3. FIG. 3 is a graph showing a plot (white dots) of
elastic modulus, E, in psi (left hand ordinate) and a plot (black dots)
R-value (right hand ordinate) versus weight percent fibers W (abscissa).
Values for conventional aerogels are represented by an X. The substantial
improvement in the mechanical properties with increasing weight percent
fibers provides additional opportunities for rigid and flexible AMCs with
no loss in the thermal insulation abilities of the materials.
The effect of compressive load on the thermal insulation values of AMCs was
also investigated. The R value of an AMC infiltrated with 12 weight
percent silica fiber was determined to be 7.3 per inch. This sample was
compressed at 27-28 psi (approximately 0.19 MPa) pressure such that the
disc thickness decreased from 0.62" to 0.5" (1.57 cm to 1.27 cm). After
the loads were removed the sample sprang back to almost the same original
thickness of approximately 0.61" (1.55 cm). The R value of this compressed
sample was measured again and was found to be unchanged by the compressive
load episodic test. This indicates that even after some mechanical damage
to the composite, the R value is retained and the structure remains
largely intact.
As indicated in Table I-Part B, gels sandwiched between fiber mat layers
(Sample C) did not show any change in R values compared with layered
fiber-gel compositions (Sample B). However, the elastic modulus of the
sandwich structure increased to 15.4 MPa (Sample C) from 6.9 MPa (Sample
B). This indicates that certain fiber-gel composition geometries might be
preferable over others.
In Table I the properties of conventional aerogels with no fibers, and
aerogel powder-fiber compacts (APFC) from the prior art are given for
comparison purposes. In comparing the properties of the materials given in
Table I, it is evident that the AMCs disclosed here are mechanically
superior to the fragile, monolithic conventional silica aerogels. Thermal
conductivity modeling analysis indicates that all AMC samples would have
relatively lower thermal conductivity compared to conventional aerogels at
temperatures higher than 26.degree. C. (See FIG. 4).
FIG. 4 is a comparison between the thermal conductivity of AMCs of the
present invention (A) and conventional aerogels (B) at temperatures higher
than 26.degree. C. In the figure, T is the temperature in .degree.K.,
.DELTA.k is the change in thermal conductivity in W/m.degree.K., and
.DELTA.R is the change in R value. The R value scale is not linear and is
given for reference only. At higher temperatures, the radiative heat flow
contribution to the conductivity increases. The lower conductivity of AMCs
at high temperatures can be explained by the opacitying effect of fiber
reinforcements to the radiative heat flow. Further, the AMCs also have
better thermal insulation values and elastic moduli compared with aerogel
powder-fiber compacts. Additionally, AMCs can be prepared with relatively
smaller amounts of raw material and significantly fewer fabrication steps
compared with APFCs.
Thus, the set of experiments described above demonstrate that due to the
introduction of fiber reinforcements in the gels prior to supercritical
drying the resultant AMCs are prepared with substantial elimination of
shrinkage and cracking during gelation and drying even when very high
temperature and pressure gradients are utilized for supercritical drying.
This results in a significantly shortened fabrication process compared
with the prior art, making it more suitable for large scale production.
The substantial improvements in the properties of the AMCs over those of
conventional aerogels and APFCs, such as no volume shrinkage, high
mechanical strength and comparable or higher thermal insulation
properties, provides a wide range of application opportunities infeasible
thus far. These types of AMCs can be produced with 0.04-0.25 g/cc
densities with a variety of reinforcement loadings, consequently changing
the mechanical properties. To further improve the thermal insulation
values the AMCs can be wrapped or packaged in metallized sheets to reduce
radiative heat flow through the sample and/or placed under soft vacuum
(0.1-0.2 atm).
EXAMPLE 2
A flexible AMC was prepared where the aerogel structure was modified by
adding silica fiber reinforcements to the silicate sol-gel solution. The
silicate sol-gel solution was prepared in manner described in Example 1.
In some cases, in order to slow down the gelation process, approximately
15 parts of CH.sub.3 OH was further added to the infiltrated silica fibers
or only 1/4 of the NH.sub.4 OH in Example 1 was added. The trapped silica
gel in the silica fiber matrix hardened in 5-10 minutes. The silica fiber
mat/sheet was rolled up in a cylindrical shape and submerged in minimum
methanol. It was then transferred to a vessel for supercritical drying
purposes above the critical point of methanol. The drying procedure
required about 250.degree. C. temperature and about 1500 psi (10.3 MPa)
pressure (see Example 1 for details of the procedure).
The same material was also prepared by using CO.sub.2 super critical drying
where methanol was first exchanged with liquid CO.sub.2. Then the gel was
dried at 40.degree. C. and 1150 psi (7.9 MPa). Later, the dried gel was
heated at 450.degree. C. to remove any residual organic or hydroxy groups
present in the aerogel. The drying process using CO.sub.2 required 1-2
days.
The dimensions of the silica fiber mat used in these experiments were
approximately 5".times.6".times.0.25" (12.7.times.15.2.times.0.6 cm) and
12".times.12".times.0.5" (30.5.times.30.5.times.1.3 cm). The flexible AMCs
contained 30-50 percent or 16-20 percent (using ridge mold) of silica
fibers by weight. The surface area of the plain silica fiber matrix was
measured to be <1 m.sup.2 /g by BET method. On the other hand, the
infiltrated silica fiber matrix indicated the BET surface area to be in
the range of 100-400 m.sup.2 /g (monolithic aerogels have typical BET
surface areas in the range 500-800 m.sup.2 /g). The bulk densities of
these infiltrated silica fiber samples were in the range of 0.09-0.13
g/cc. As observed by SEM (scanning electron microscopy) the porous spaces
in the silica fiber matrix were almost completely filled by the
infiltrated monolithic aerogel material.
Thermal property measurements, including thermal conductivity and thermal
resistance were carried out for these materials. In addition, the heat
capacity was also calculated from the differential scanning calorimetry
(DSC) data. The thermal conductivity of the silica fiber infiltrated with
silica aerogel were determined to be in the range of 0.018 to 0.020
W/m.degree.K. This thermal conductivity value is equivalent to 0.125-0.142
BTU.inch/hr.ft.sup.2.F and R value of 7 to 8. For comparison, the thermal
conductivities (also R values) of glass wool insulation and CFC-based
polyurethane refrigeration insulation are 0.29 (R=3.44) and 0.142
BTU.inch/hr.ft.sup.2 .degree.F. (R=7), respectively. Hence, both rigid and
flexible infiltrated aerogel insulations are significantly better than
current fiber glass insulation and are at least as good as CFC-filled
polyurethane foam insulation materials. Unlike CFC-polyurethane foams,
however, the flexible AMCS would potentially retain these R values
indefinitely.
EXAMPLE 3
Same as Example 1, but the long fiber reinforcements were replaced with
shorter fibers or whiskers. Hence, the resultant AMCs were fiber or
whisker reinforced. If these fibers/whiskers are dispersed under
microgravity the fiber/whisker distribution in the aerogel matrix would be
more uniform. The fibers are preferably about 5 to 200 um in diameter and
the whiskers are preferably about 0.5 to 1.0 um in diameter and about 25
to about 100 um long. The fibers and whiskers are preferably inorganic
oxides or metals.
EXAMPLE 4
The process of Example 1 was repeated but the fiber matrix was replaced by
an alumina ceramic foam. The silicate solution penetrated the pores well
and gelled in the pores. The gel was converted into an aerogel after
supercritical drying.
EXAMPLE 5
The process of Example 1 is repeated but the fiber matrix is replaced by a
honeycomb structure. The process of the invention combined with this type
of structure provides extraordinary mechanical properties.
EXAMPLE 6
The process of Example 1 is repeated, however, the silicate gel is
reinforced by particles prior to gelation and supercritical drying. These
particles can be metal and inorganic powders.
EXAMPLES 7A and 7B
These examples are the same as Example 1, except that the formulations for
silica aerogel were different. Since TMOS is not available in commercial
quantities and is more expensive, a cheaper alternative was sought. For
Example 7A this new composition involved tetraethoxy orthosilicate (TEOS)
and methanol. Methanol was selected as a preferred alcohol based on the
lower cost for commercial quantities. However, other alcohols such as
ethanol can also be used. The procedure for preparing this gel can be
described by these two reactions:
##STR1##
The molar ratio of Si(OC.sub.2 H.sub.5).sub.4 :H .sub.2 O:HCl:NH.sub.4
OH:CH.sub.3 OH is equal to 1:3:0.0007:0.00124:3.75. In the first step only
1 mole of water/mole silicon was added along with methanol and HCl acid.
This mixture was stirred and heated at 60.degree. C. for 1 hour. In the
second step a mixture of left over water and ammonia base was added at
0.degree.-5.degree. C. to slow down the condensation reaction. This gel
was slightly cloudy as compared to TMOS gels. The bulk densities of the
AMCs produced from TEOS/CH.sub.3 OH were found to be more dense (i.e.
1.1-1.8 times) than those of TMOS AMCs.
For Example 7B a similar silica gel were prepared using acetic acid and the
reactions are represented below:
##STR2##
The molar ratios of Si(OC.sub.2 H.sub.5).sub.4 :H.sub.2 O:CH.sub.3
COOH:NH.sub.4 OH:CH.sub.3 OH equal 1.00:10.09:0.39:0.01:16.82. In the
first step only acetic acid, water and methanol were added to TEOS. This
mixture was stirred and heated at 60.degree. C. for 1 hour. In the second
step, a mixture of ammonia base and methanol was added to the silicate
solution. This gel was quite clear, though large volume shrinkage was
observed for aerogels made by this method. Shrinkage could be reduced by
optimizing the composition and the reaction conditions.
EXAMPLES 8A-8E
Same as Examples 2 through 6, except that the formulation for silica
aerogel is different. Since TMOS is not available in commercial quantities
and is more expensive, a cheaper alternative is preferred. This new
composition involves tetraethoxy orthosilicate (TEOS) and methanol.
Methanol is selected as a preferred alcohol based on the lower cost for
commercial quantities. However, other alcohols such as ethanol can also be
used. The procedure for preparing this gel can be described by the same
reactions as in Example 7A. The molar ratio of Si(OC.sub.2 H.sub.5).sub.4
: H.sub.2 O:HCl:NH.sub.4 OH:CH.sub.3 OH is equal to 1:3:0.0007:0.00124:
3.75. In the first step only 1 mole of water/mole silicon is added along
with methanol and HCl acid. This mixture is stirred and heated at
60.degree. C. for 1 hour. In the second step a mixture of left over water
and ammonia base is added at 0.degree.-5.degree. C. to slow down the
condensation reaction. This gel is slightly cloudy as compared to TMOS
gels. This probably results from a higher degree of agglomeration, hence,
scattering. The bulk densities of the AMCs produced from TEOS/CH.sub.3 OH
should be more dense (i.e. 1.1-1.8 times) than those of TMOS AMCs.
EXAMPLES 9A-9E
Same as Examples 8A through 8E, except that a similar silica gel is
prepared using acetic acid and the reactions are represented by the same
equations as in Example 7B. The molar ratios of Si(OC.sub.2 H.sub.5).sub.4
:H.sub.2 O:CH.sub.3 COOH:NH.sub.4 OH:CH.sub.3 OH equal 1.00:10.09:
0.39:0.01:16.82. In the first step only acetic acid, water and methanol
are added to TEOS. This mixture is stirred and heated at 60.degree. C. for
1 hour. In the second step, a mixture of ammonia base and methanol is
added to the silicate solution. This gel is quite clear, though large
volume shrinkage is expected for aerogels made by this method unless the
composition and the reaction conditions are optimized.
EXAMPLE 10
Same as Example 1, except that the silica aerogel was made from aqueous
sodium silicate solution. In preparing an aqueous silica gel, a solution
of sodium silicate in distilled water was made such that the specific
gravity of the solution is between 0.99-1.15 g/cc. To 60 ml aqueous sodium
silicate solution, 10 ml of 10 weight percent HCl was added while
stirring. This results in gelation within a few minutes to an hour. The
gel was then supercritically dried as before. The liquid water in the
first sol can be either first exchanged with alcohol then supercritically
dried above the critical point of the alcohol or the water can be directly
supercritically dried.
EXAMPLES 11-19
The method of Example 10 is repeated for Examples 2 through 9, except that
the silica aerogel is made from aqueous sodium silicate solution.
EXAMPLES 20-25
Same as Examples 1 through 6, except that the aerogel composition is
ZrO.sub.2, TiO.sub.2, Al.sub.2 O.sub.3, and other similar oxides. These
are prepared from their corresponding alkoxide precursors and alcohols. In
systems like ZrO.sub.2 and Al.sub.2 O.sub.3 where agglomeration and
precipitation is a common problem, acetyl acetone and other additives are
added to prepare a homogeneous gel.
EXAMPLES 26-31
Same as Examples 1 through 6, but the silica aerogel is replaced by organic
aerogels made from formaldehyde and resorcinol, or resorcinol and
melamine. The preparation of these organic aerogels without gel
reinforcement is described by Pekala et. al. in Mat. Res. Soc. Symp. Vol.
180, 791-795, 1990. The organic liquids are finally replaced with liquid
CO.sub.2 or other similar solvents for supercritical drying purposes. This
composition results in organic aerogel-inorganic fiber composites.
EXAMPLES 32-36
Same as Examples 2 through 6, but the silica aerogel is replaced by
organic-inorganic composite aerogels. These composite aerogels can be
prepared by starting with TMOS where one or more alkoxide groups are
replaced by inert organic groups. These inert organic groups include alkyl
or aryl chains. On the other hand, if the organic group is reactive,
simultaneous polymerization of inorganic and organic groups would result
in an inorganic-organic composite. This type of composition introduces
more flexibility into the composite due to the presence of organic
moieties and polymers.
EXAMPLE 37
Same as Example 1, except that a surfactant (e.g. Tergitol XH) was added to
the sol-gel solution to reduce the surface tension of the liquid. The
reduced surface tension eased the liquid removal from the gel and improved
the structural strength of the aerogel material.
EXAMPLES 38-72
Same as Examples 2 through 36, except that a surfactant (e.g. Tergitol XH)
is added to the sol-gel solution to reduce the surface tension of the
liquid. The reduced surface tension eases the liquid removal from the gel
and improves the structural strength of the aerogel material.
EXAMPLE 73
Same as Example 1, except the monolithic AMC was made in the shape of a
disc, and the thickness of the disc was less than 1 mm. Due to low
pressure drop of the thin aerogel membrane samples and improved mechanical
strength of AMCs these can be used as gas filtration/purification and
separation media.
EXAMPLES 74-143
Same as Examples 2-72, except the monolithic AMC is made in the shape of a
disc, and the thickness of the disc is less than 1 mm. Due to low pressure
drop of the thin aerogel membrane samples and improved mechanical strength
of AMCs these can be used as gas filtration/purification and separation
media.
The disclosed AMCs have improved mechanical strength, relatively good
moisture resistance, a range of flexibility, and good thermal resistance.
The processes developed for these AMCs has been shortened to 3-7 hours and
the solvent was totally recycled. The flexibility of this material makes
it possible to easily replace insulation material for houses and for
insulation in jacket form in applications up to 700.degree. C. Apart from
these applications the material can also replace the insulation material
currently used in refrigerators. These foam materials are currently
chloro-fluoro carbon blown polyurethane foams which are environmentally
damaging, whereas, aerogels are environmentally safe and have extremely
low thermal conductivities. Monolithic aerogels, for example, have thermal
conductivities in the range of 0.01-0.02 W/m.degree.K. as compared to the
thermal conductivity of pyrex glass wool, 0.0418 W/m.degree.K. The present
system will provide a good combination of flexibility and thermal
conductivities for insulation purposes. In addition, this system being an
inorganic oxide, is also nonflammable. The other applications of this
material may include packaging, comforters, and other thermally efficient
apparel.
While the forms of the invention herein disclosed constitute presently
preferred embodiments, many others are possible. It is not intended herein
to mention all of the possible equivalent forms or ramifications of the
invention. It is to be understood that the terms used herein are merely
descriptive, rather than limiting, and that various changes may be made
without departing from the spirit or scope of the invention.
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